1. Field of the Invention
The present invention relates generally to satellite based positioning systems (SATPS) such as the Global Positioning System (GPS).
2. Description of Related Art
A SATPS receiver generally determines its positions by triangulating its Line of Sight (LOS) range to several satellites or space vehicles. A GPS receiver, for example, computes a 4-dimensional solution involving latitude, longitude, altitude, and time using the LOS ranges to as few as four satellites. The accuracy of the solution is a direct function of the accuracy of the range measurements.
SATPS receivers are growing rapidly in popularity and application. GPS receivers, for example, are now common in aviation, marine, and terrestrial applications. An increasingly common terrestrial application for GPS receivers is in automobiles. In the automotive context, the vehicle's location is typically displayed on an electronic display of a street map. It is vital in this context, therefore, to provide the driver with continuously updated position solutions, collectively called a "ground track," that accurately track the vehicle's movement from one moment to the next. From a usability point of view, the groundtrack should only reflect the vehicle's movement. In fact, experience shows that consumers consider groundtrack fidelity as one of the most important criteria in choosing a receiver. It is extremely important, therefore, that the groundtrack displayed on the GPS receiver's electronic map not have spurious jumps, stair steps, spikes, jigs, or jogs that are unrelated to the vehicle's actual path.
There are a number of factors, however, which may cause discontinuities in the position solutions which make up the groundtrack on the display of an automotive SATPS receiver.
One given source of position solution discontinuities is "Selective Availability" (SA) which is used to restrict the accuracy of civilian GPS receivers to roughly 100 meters 1-sigma for purposes of national security. The Department of Defense (DOD) implements SA by purposely injecting error into the satellite range signals.
Another common source of position solution discontinuities is "multipath," the phenomenon where the main LOS signal from a given satellite reaches the GPS receiver's antenna and is followed by additional "copies" due to reflection from nearby objects, such as buildings or cliffs. The multipath effect is particularly troublesome for automotive receivers because they are frequently used in cities and surrounded by tall buildings. This environment is sometimes called an "urban canyon" due to the canyon-like setting it resembles. Regardless of source, Multipath can be a very vexing problem because the additional signals may be very strong, but very wrong.
Yet another source of position solution discontinuities is the fact that the SATPS receiver may see a different constellation of satellites from one moment to the next. If the GPS receiver is in motion in an urban canyon environment, for example, individual satellites may become blocked and unblocked by nearby buildings. There is always some degree of error in a position solution based on one group of satellites. If the position solution is suddenly based on another group, the result may be a jump or discontinuity in position relative to the solution computed with the first group. This will, of course, cause visual groundtrack discontinuities on the receiver's display.
Other approaches to groundtrack smoothing known to this inventor, such as those based on Digital Signal Processors, are overly complicated and relatively expensive.
There remains a need, therefore, for a simple, cost effective method of smoothing resultant values in Kalman Filter systems for use in, for example, mitigating multipath effects and smoothing groundtracks in a mobile SATPS receiver.